1. Field of the Invention
The present invention relates to an optical device, notably for a motor vehicle, such as a lighting and/or signaling and/or interior lighting device, notably having a photometric function which is helpful for the use of the vehicle on the road in that it enables the vehicle to be seen by other vehicles or enables the driver of said vehicle to see outside the vehicle.
2. Description of the Related Art
In the field of signaling, as well as in that of lighting, numerous regulatory constraints allow little scope for changes in the appearance of the lights in the illuminated condition, since the photometric characteristics of the light beams are very closely specified. However, style and aesthetic factors are very important features of this type of product, and vehicle parts manufacturers try to provide their products with a “signature” which makes them easily recognizable by the end user.
It is known to use surface light sources to provide lighting and/or signaling and/or interior lighting functions for motor vehicles. A new type of surface light source is being developed at present, in the form of organic light-emitting diodes. It would be helpful to use these to provide lighting and/or signaling functions. However, these sources have some drawbacks. The degrees of directivity achieved at present are of the form (cos θ)11, where θ represents the emission angle with respect to the normal to the emission surface and (cos θ)11 represents the intensity of the light emitted in the direction θ relative to the intensity emitted in the direction of the normal to the surface. This degree of directivity is insufficient for the effective provision of certain signaling functions, notably a brake signaling function. This is because, in order to provide this signaling function, greater directivity is required in the vertical plane; in other words, the light emitted by the diode must be less diffused vertically.
To overcome this drawback, there are known organic light-emitting diodes that have a layer on their emitting surface for modifying their directivity. In this way, a directivity of the form (cos θ)15 is achieved, where θ represents the emission angle with respect to the normal to the emission surface and (cos θ)15 represents the degree of illumination in the direction θ. By contrast with the situation described previously, this solution results in a directivity in the horizontal plane which is too great for the provision of a brake-type signaling function.
Consider a rectangular organic light-emitting diode having an emitting surface of 5 mm by 220 mm, positioned perpendicularly to an optical axis and having a global emission indicator of cos11 times the angle of observation with respect to its normal. This gives us the distribution at infinity shown in FIG. 1 (for an arbitrary flux of 50 Im). The horizontal and vertical sections are identical and have a profile varying as cosine11. One of these sections is shown in FIG. 2. With this system, we obtain the photometric grid shown in FIG. 11. This photometric grid does not conform to the standardized photometric grid for a brake-type signaling device. This is because the luminous intensity emitted, notably, in the vicinity of the optical axis is insufficient when a large amount of light is emitted unprofitably above 15° upwards and below 15° downwards.
Additionally, the levels of luminance produced by organic light-emitting diodes are limited. It is therefore necessary to provide extended emission areas in order to obtain a lighting and/or signaling function.